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Abstract:

An illumination system and a projection objective of a mask inspection
apparatus are provided. During operation of the mask inspection
apparatus, the illumination system illuminates a mask with an
illumination bundle of rays having a centroid ray that has a direction
dependent on the location of the incidence of the illumination bundle of
rays on the mask.

Claims:

1. An illumination system of a mask inspection apparatus, wherein the
illumination system is configured to, in operation of the mask inspection
apparatus, illuminate a mask with an illumination bundle of rays which
has a centroid ray; and wherein the illumination system is further
configured such that, during the operation of the mask inspection
apparatus, said centroid ray has a direction dependent on the location of
the incidence of the illumination bundle of rays on the mask.

2. The illumination system of claim 1 in which the maximum angle between
the planes of incidence of two bundles of rays incident on different
locations of the mask is at least 3.degree..

3. The illumination system of claim 1 in which the maximum angle between
the centroid rays of two bundles of rays incident on different locations
of the mask is at least 3.degree..

4. The illumination system of claim 1 in which a mask that is analyzed in
the operation of the mask inspection apparatus is designed for use with
an illumination region in the form of a segment of a ring in a projection
exposure apparatus.

5. The illumination system as of claim 4 in which the variation in the
direction of the centroid ray, that is dependent on the location of
incidence of the illumination bundle of rays on the mask, complies with
the condition: sin ( α ) = y R ##EQU00005## wherein: x, y
denote co-ordinates of the mask plane, R denotes the radius of the ring
field, and α denotes the angle between the plane of incidence,
formed by the centroid ray which is incident on the mask from the
illumination system and that which is reflected by the mask, with the y=0
plane.

6. The illumination system of claim 1 in which the illumination system is
designed for operation in an extreme ultra-violet (EUV) mode.

7. The illumination system of claim 1 in which the variation in the
direction of the centroid ray, that is dependent on the location of
incidence of the illumination bundle of rays on the mask, is such that
the magnitude of the angle between the centroid ray and the surface
normal on the mask is maintained.

8. The illumination system of claim 1 in which there is provided at least
one blade movable in a predetermined plane of movement for adjustment of
the variation in the direction of the centroid ray, that is dependent on
the location of incidence of the illumination bundle of rays on the mask.

9. An illumination system of a mask inspection apparatus, wherein the
illumination system is configured to, in operation of the mask inspection
apparatus, illuminate a mask with an illumination bundle of rays which
has a centroid ray; wherein the illumination system is further configured
such that, during the operation of the mask inspection apparatus, said
centroid ray has a direction dependent on the location of the incidence
of the illumination bundle of rays on the mask, wherein the illumination
system comprises at least one blade movable in a predetermined plane of
movement for adjustment of the variation in the direction of the centroid
ray during the operation of the mask inspection apparatus, dependent on
the location of incidence of the illumination bundle of rays on the mask.

10. The illumination system of claim 9 in which said plane of movement
extends in substantially coplanar relationship with the mask plane.

11. The illumination system of claim 9 in which a region of the plane of
movement, over which the blade is movable for adjustment in the variation
in the direction of the centroid ray, that is dependent on the location
of incidence of the illumination bundle of rays on the mask, has a
substantially reniform contour.

12. The illumination system of claim 9 in which the blade is adapted for
adjustment of at least one of quadrupole illumination setting, dipole
illumination setting, annular illumination setting, or conventional
illumination setting.

13. The illumination system of claim 9 in which the blade is arranged
rotatably.

14. The illumination system of claim 9 in which the blade is in the form
of a variably adjustable blade arrangement, wherein by adjustment of said
blade arrangement a substructure can be afforded in the brightness
distribution of an illumination pupil of an extreme ultra-violet (EUV)
projection exposure apparatus in the mask inspection apparatus.

15. The illumination system of claim 14 in which said blade arrangement
has at least two blades movable relative to each other.

16. The illumination system of claim 15 in which said blades have at
least one of blade openings of differing shape or blade openings of
differing sizes.

17. The illumination system of claim 15 in which at least one of said
blades has an apodising arm.

18. The illumination system of claim 9 in which the mask is arranged
rotatably.

19. A projection objective of a mask inspection apparatus, wherein the
projection objective is configured to, in operation of the mask
inspection apparatus, observe a mask with an observation bundle of rays
having a principal ray, and wherein the projection objective is further
configured such that, in the operation of the mask inspection apparatus,
said principal ray has a direction dependent on the starting location of
the observation bundle of rays on the mask.

20. The projection objective of claim 19 in which the projection
objective is designed for operation in an extreme ultra-violet (EUV)
mode.

21. The projection objective of claim 19 in which there is provided at
least one blade which is movable in a predetermined plane of movement for
adjustment of the variation in the direction of the principal ray, that
is dependent on the starting location of the observation bundle of rays
on the mask.

22. The projection objective of claim 21 in which the region of the plane
of movement, over which the blade is movable for adjustment of the
variation in the direction of the principal ray, that is dependent on the
starting location of the observation bundle of rays on the mask, has a
substantially reniform contour.

23. The projection objective of claim 21 in which said plane of movement
extends in substantially coplanar relationship with the mask plane.

24. A mask inspection apparatus comprising: an illumination system
configured to, in operation of the mask inspection apparatus, illuminate
a mask with an illumination bundle of rays which has a centroid ray,
wherein the illumination system is further configured such that, during
the operation of the mask inspection apparatus, said centroid ray has a
direction dependent on the location of the incidence of the illumination
bundle of rays on the mask; and a projection objective configured to, in
the operation of the mask inspection apparatus, observe a mask with an
observation bundle of rays having a principal ray, wherein the projection
objective is further configured such that, in the operation of the mask
inspection apparatus, said principal ray has a direction dependent on the
starting location of the observation bundle of rays on the mask.

25. The mask inspection apparatus of claim 24 in which a blade of the
illumination system and a blade of the projection objective are movable
synchronously relative to each other in opposite directions.

26. A method of operating a mask inspection apparatus, comprising:
illuminating, using an illumination system, a mask with an illumination
bundle of rays having a centroid ray, and observing, using a projection
objective, said mask with an observation bundle of rays having a
principal ray, wherein, during a movement of the mask in the course of
mask inspection, the direction of the centroid ray and the direction of
the principal ray are respectively varied in dependence on the location
on the mask.

27. An illumination system of a mask inspection apparatus wherein the
illumination system comprises at least one blade arrangement which is
variably adjustable in such a way that a substructure can be afforded in
the brightness distribution of an illumination pupil of an extreme
ultra-violet (EUV) projection exposure apparatus by adjustment of said
blade arrangement.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of and claims priority to
International patent application PCT/EP2011/052184, filed on Feb. 15,
2011, which claims priority to U.S. provisional patent application
61/306,624, filed on Feb. 22, 2010 and German patent application 10 2010
009 022.0, filed on Feb. 22, 2010. Each of these applications is
incorporated by reference in its entirety.

FIELD

[0002] The disclosure concerns an illumination system and a projection
objective of a mask inspection apparatus.

BACKGROUND

[0003] Microlithography is used for the production of microstructured
components such as, for example, integrated circuits or liquid crystal
displays (LCDs). The microlithography process is carried out in what is
referred to as a projection exposure apparatus having an illumination
system and a projection objective. In that case the image of a mask (also
referred to as a reticle) illuminated by the illumination system is
projected by means of the projection objective on to a substrate (for
example, a silicon wafer) which is coated with a light-sensitive layer
(photoresist) and arranged in the image plane of the projection objective
in order to transfer the mask structure on to the light-sensitive coating
on the substrate. Mirrors are used as optical components for the imaging
process in projection objectives designed for the extreme ultra-violet
(EUV) range, that is to say at wavelengths of, for example, about 13 nm
or about 7 nm, due to the lack of availability of suitable translucent
refractive materials.

[0004] In the lithography process unwanted defects on the mask have a
particularly detrimental effect as they can be reproduced with each
illumination step and there is thus the danger that in the worst-case
scenario the entire production of semiconductor components is unusable.
It is therefore a matter of great significance for the mask to be checked
for adequate imaging capability before use thereof in mass production. In
that respect the changeover from vacuum ultra-violet (VUV) systems to EUV
systems is linked not only to changes in the materials and process steps
used, but in particular also to a higher level of sensitivity (typically
by four times) of the reflectively designed EUV mask, in relation to
topological defects, in comparison with conventional VUV masks.

[0005] In that respect the problem which inter alia arises in practice is
that, depending on the respective form of the defects and the position
thereof relative to the structure to be imaged, in the mask, deviations
which are difficult to predict occur in the imaging process. Therefore
direct analysis of the imaging effect of possible defect positions is
desirable for minimizing the mask defects and for implementing successful
mask repair. There is thus a need for the mask to be quickly and easily
tested, more specifically as far as possible under the same conditions as
occur really in the projection exposure apparatus. It is to be observed
in that respect that different degrees of coherence of the light,
different illumination settings and greater and greater numerical
apertures involving values of NA=0.35 and above are set in the
illumination system of current EUV systems, which in practice represents
a demanding challenge in terms of emulation or reproduction of the
imaging procedure of the projection exposure apparatus in mask
inspection.

SUMMARY

[0006] The disclosure provides an illumination system and a projection
objective of a mask inspection apparatus, which permit more accurate
emulation of the conditions occurring in the projection exposure
apparatus.

[0007] In an aspect, the disclosure concerns an illumination system of a
mask inspection apparatus, wherein the illumination system in operation
of the mask inspection apparatus illuminates a mask with an illumination
bundle of rays which has a centroid ray, wherein said centroid ray has a
direction dependent on the location of the incidence of the illumination
bundle of rays on the mask.

[0008] In that respect the term "centroid ray" is used in accordance with
the usual terminology to denote the energy mean over all subrays of a
beam.

[0009] The centroid ray of the illumination bundle of rays is of a
direction dependent on the location of the incidence of the illumination
bundle of rays on the mask. This makes it possible for the pupil
illumination of the projection objective to be more exactly emulated. For
greater understanding attention is firstly directed to FIGS. 1 through 3
to explain the underlying concept of the present disclosure.

[0010] The reference to a variation in the direction of the centroid ray
in dependence on the location on the mask is preferably used to denote a
variation in respect of which the maximum angle between the centroid rays
of two bundle of rays which are incident on different locations on the
mask is at least 1°. In some embodiments, the maximum angle
between the centroid rays of two bundles of rays incident on different
locations on the mask is at least 3°, in particular at least
5°, further particularly at least 10° and further
particularly at least 15°.

[0011] In another approach the maximum angle between the planes of
incidence of two bundles of rays incident on different locations on the
mask is at least 3°, in particular at least 5°, further
particularly at least 10° and further particularly at least
15°.

[0012] In an embodiment a mask which is analyzed in operation of the mask
inspection apparatus is designed for use with an illumination region, in
the shape of a segment of a ring, in a projection exposure apparatus.

[0013] In a lithography process the entire mask is scanned by a scanner
slot which in the case of conventional (VUV) systems is typically of a
rectangular geometry but which in the case of EUV systems is of a
geometry in the shape of a segment of a ring. The background here is that
the EUV optical system has a rotationally symmetrical optical design, but
in the lithography process only a small portion of that annular field is
used, which is comparatively far away from the optical axis. In addition,
when using a reflective EUV mask, it is known that to separate the
illumination system and the projection objective it is necessary for the
illumination bundle of rays to be directed at a finite angle of incidence
on to the mask or the reticle, in which case (without the disclosure
being restricted thereto) a typical angle in respect of the centroid ray
relative to the surface normal on the mask can be 6° in present
scanners.

[0014] FIGS. 1 through 3 each show an object field 110 in the shape of a
segment of a ring, on a mask 101 having a region 102 with structures to
be imaged. In FIG. 1 the position which is currently being illuminated is
at the center of the object field 110, whereas the position which is
currently being illuminated in FIG. 2 is in the region of the left-hand
edge of the object field 110 and in FIG. 3 it is in the region of the
right-hand edge thereof.

[0015] Consequently the projection objective also receives the light or
observation bundle of rays with a principal ray angle (or chief ray
angle) which is of the same value in relation to the surface normal of
the mask 101. As now the optical system in the scanner and in particular
in the projection optical system is of a rotationally symmetrical
configuration, that angle, for reasons of symmetry, must always be in
that plane formed by the object point which is just being observed of the
mask 101 (or the object plane of the projection optical system), with the
optical axis.

[0016] The bundle of rays reflected at the object point which is
respectively being observed is also incident in the projection objective
at a (principal ray) angle (in the present example, 6°), but that
angle is now so oriented that it is in that plane that is formed with the
optical axis of the projection objective. If now as shown in FIGS. 1
through 3 various points are considered along the object field 110, in
the shape of a segment of a ring, on the mask 101, then the rays passing
into the projection objective do not all extend approximately parallel to
each other, but rather are always perpendicular to the object field 110
in the shape of a segment of a ring, as can be seen from FIGS. 1 through
3.

[0017] A comparison between the situations in FIGS. 2 and 3 and the
situation in FIG. 1 accordingly shows that, for the left-hand and
right-hand positions of the object field 110 respectively, the plane that
the illumination centroid ray forms with the (principal) ray passing from
the mask 101 into the projection objective (that is to say the plane from
the bundle of rays incident in the mask 101 and that reflected by the
mask 101) is turned in comparison with the situation in FIG. 1 (in which
the illuminated position is at the center of the object field 110),
wherein it always includes the optical axis of the projection exposure
apparatus or the scanner. Thus a movement along the object field 110 in
the shape of a segment of a ring requires the above-described plane also
to be turned therewith.

[0018] In an embodiment the variation in the direction of the centroid
ray, that is dependent on the location of incidence of the illumination
bundle of rays on the mask, complies with the following condition:

sin ( α ) = y R , ##EQU00001##

wherein: [0019] x, y denote the co-ordinates of the mask plane, [0020] R
denotes the radius of the ring field, and [0021] α denotes the
angle between the plane of incidence (formed by the centroid ray which is
incident on the mask from the illumination system and the centroid ray
reflected by the mask) with the y=0 plane.

[0022] In an embodiment the illumination system is designed for operation
in the EUV mode.

[0023] Although in examples described in the disclosure implementation is
effected in a mask inspection apparatus designed for EUV, the disclosure
is not restricted thereto. Thus the disclosure can also be applied to a
mask inspection apparatus of a higher working wavelength (for example 193
nm) as angle differences in illumination can also occur in relation to
such wavelengths so that improved mask inspection can possibly also be
achieved by the variation according to the disclosure in the direction of
the centroid ray in dependence on the location on the mask.

[0024] In some embodiments, systems involving a convergent configuration
for the principal rays (that is to say the principal rays pass after
reflection at the mask towards the optical axis) can be used. In some
embodiments, systems involving divergent principal rays can be used.
Systems in which the principal rays pass divergently into the projection
objective are known for example from US 2005/0088760 A1 (see FIGS. 91 and
93 of US 2005/0088760 A1).

[0025] In some embodiments, the variation in the direction of the centroid
ray, that is dependent on the location of incidence of the illumination
bundle of rays on the mask, is such that the magnitude of the angle
between the centroid ray and the surface normal on the mask is
maintained.

[0026] In some embodiments, there is provided at least one blade (or
aperture stop) which is movable in a predetermined plane of movement to
adjust the variation in the direction of the centroid ray, that is
dependent on the location of incidence of the illumination bundle of rays
on the mask.

[0027] In embodiments of the disclosure the blades discussed here and
hereinafter can be in the usual way in the form of disks or plates which
are provided with holes designed in accordance with the desired
illumination setting and which in other respects are non-transmitting.
The disclosure however is not restricted thereto. Thus in further
embodiments, in place of apertured disks or plates, it is also possible
to use partially transmitting and/or partially polarizing components.
That is advantageous for example if a variation in the transmission of
the scanner as a function of the pupil co-ordinates is to be emulated or
in illumination the variation in the illumination strength is to be
adjusted over the illumination pupil or also for the emulation of
polarizing elements in the (EUV) scanner or the projection exposure
apparatus. For continuous adjustment of the numerical aperture the blade
itself can also be variable in its shape, for example by virtue of an
iris (or iris blade).

[0028] In addition, in place of a physical blade, it is also possible to
use another suitable device for adjusting the direction of incidence of
the illumination rays, for example a per se known multi-mirror array
(referred to as an MMA), with a multiplicity of (micro)mirror elements
which are adjustable differently from each other.

[0029] In an embodiment the above-mentioned predetermined plane of
movement is in substantially coplanar relationship with the plane of the
mask.

[0030] In an embodiment that region of the plane of movement, over which
the blade is movable for adjustment of the variation in the direction of
the centroid ray, that is dependent on the location of incidence of the
illumination bundle of rays on the mask, has a substantially reniform
contour.

[0031] In an embodiment the blade is designed for adjustment of one or
more of the following illumination settings; quadruple illumination
setting, dipole illumination setting, annular illumination setting,
conventional illumination setting. In that respect in accordance with the
usual terminology the term conventional illumination setting is used to
denote circular illumination with an intensity which is as uniform as
possible within the circle.

[0032] In an embodiment the blade is arranged rotatably, which can be
advantageous depending on the respective specific configuration of the
projection exposure apparatus to be emulated.

[0033] In an embodiment the blade is in the form of a variably adjustable
blade arrangement, wherein a substructure can be preserved in the
brightness distribution of an illumination pupil of an EUV projection
exposure apparatus, by adjustment of that blade arrangement.

[0034] In an embodiment the blade arrangement has at least two blades
movable relative to each other. Those blades can have in particular blade
openings of differing shape and/or sizes. Alternatively or additionally
one of those blades can have an apodising arm.

[0035] In an embodiment the mask is arranged rotatably. In that case a
field-dependent displacement of the plane of the illumination system and
possibly the projection objective is only still required to adjust
different angles of inclination (for example 6°, 9°, etc.).

[0036] In a further aspect the disclosure also concerns a projection
objective of a mask inspection apparatus, wherein in operation of the
mask inspection apparatus the projection objective observes a mask with
an observation bundle of rays having a principal ray, wherein that
principal ray has a direction dependent on the starting location of the
observation bundle of rays on the mask.

[0037] In an embodiment there is provided at least one blade movable in a
predetermined plane of movement for adjustment of the variation in the
direction of the principal ray, that is dependent on the starting
location of the observation bundle of rays on the mask.

[0038] In an embodiment that region of the plane of movement, over which
the blade is movable for adjustment of the variation in the direction of
the principal ray, that is dependent on the starting location of the
observation bundle of rays on the mask, has a substantially reniform
contour.

[0039] In an embodiment that plane of movement extends in substantially
coplanar relationship with the plane of the mask.

[0040] In a further aspect the disclosure also concerns a mask inspection
apparatus having an exposure system and a projection objective having the
above-described features.

[0041] In that respect in an embodiment a blade of the illumination system
and a blade of the projection objective are movable in mutually
synchronous relationship in opposite directions.

[0042] In a further aspect the disclosure also concerns a method of
operating a mask inspection apparatus, wherein in operation of the mask
inspection apparatus the illumination system illuminates a mask with an
illumination bundle of rays having a centroid ray and wherein the
projection objective observes said mask with an observation bundle of
rays having a principal ray, wherein the direction of the centroid ray
and the direction of the principal ray are respectively varied in
dependence on the location on the mask.

[0043] In a further aspect of the disclosure account is also taken of
situations in which the illumination pupil, in its brightness
distribution, has substructures deviating from an idealized form (which
typically forms the basis in respect of dipole or quadrupole illumination
settings). Such a substructure can be governed in particular by using in
the illumination system a honeycomb condenser generating a multiplicity
of light passages which are superposed in the plane of the mask. Those
light passages now do not completely fill the pupil so that it is
possible to see individual spots or "illumination peaks".

[0044] To take account of that it is also possible in accordance with the
disclosure to produce in the plane of the blade not only for example
idealized or homogenous dipoles or quadrupoles in the illumination
setting, but instead to preserve within the plane of the blade the
complete "substructured" illumination distribution.

[0045] In accordance with an aspect therefore the disclosure concerns an
illumination system of a mask inspection apparatus, wherein the
illumination system has at least one blade arrangement which is variably
adjustable in such a way that, by adjustment of that blade arrangement, a
substructure can be preserved in the brightness distribution of an
illumination pupil of an EUV projection exposure apparatus.

[0046] In an embodiment the blade arrangement has at least two blades
movable relative to each other. Those blades can have in particular blade
openings of differing shape and/or sizes. Alternatively or additionally
one of those blades can have an apodising arm.

[0047] Further configurations of the disclosure are to be found in the
description and the appendant claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The disclosure is described in greater detail hereinafter by
examples illustrated in the accompanying drawings in which:

[0049] FIGS. 1-5 show diagrammatic views to illustrate and explain the
principle of the present disclosure,

[0050] FIG. 6 shows a diagrammatic view of a mask inspection apparatus
which is designed for EUV and in which the disclosure is carried into
effect,

[0051] FIGS. 7a-7b show further diagrammatic views to explain the
disclosure, and

[0052] FIGS. 8-11 show diagrammatic views to explain embodiments in
accordance with a further aspect of the disclosure.

DETAILED DESCRIPTION

[0053] Hereinafter reference is made to FIGS. 4 through 7 to describe how,
in a mask inspection apparatus in accordance with an example of the
disclosure, the above-described illumination conditions which are
implemented by the projection exposure apparatus or the scanner (that is
to say the variation in the direction of the light incident from the
illumination system and the light collected by the projection optical
system) are reproduced as well as possible.

[0054] As is only diagrammatically illustrated in FIG. 6 the mask
inspection apparatus 600 includes an illumination system 610 and a
projection objective 620, wherein light of an EUV light source 601 passes
into the illumination system 610 and an illumination bundle of rays 615
is directed on to a respectively illuminated region or the object field
of a mask 630 arranged in the object plane of the projection objective
620, and wherein that object field is imaged (by way of an observation
bundle of rays 625) on to a camera (CCD sensor arrangement) 640 by the
projection objective 620.

[0055] In accordance with the embodiment described by way of example with
reference to FIGS. 4 through 7, a respective blade 650 and 660 is used
both in the illumination system 610 and also in the projection objective
620 of the mask inspection apparatus 600, as described hereinafter; the
respective blade 650 and 660 respectively is movable in a predetermined
plane of movement to adjust the variation in the direction of the
centroid ray, that is dependent on the location of the incidence of the
illumination bundle of rays 615 on the mask 630.

[0056] Those blades 650, 660 are in turn preferably respectively placed in
a pupil plane and respectively moved in a lateral direction (that is to
say in the x-direction and the y-direction in the illustrated co-ordinate
system).

[0057] As described hereinafter with reference to FIGS. 4 and 5 the
respective centroid ray is influenced by means of blades 650, 660 in the
illumination system 610 and the projection objective 620 respectively. In
that case, the angle of the illumination optical system (that is to say
the angle of the centroid ray of the illumination bundle of rays incident
on the mask) is adjusted by way of the position of the blade 650 provided
in respect of the illumination system 610, and the angle of the centroid
ray or main ray of the (observation) bundle of rays in the imaging ray
path, which passes into the projection objective, is adjusted by way of
the position of the blade 660 provided in respect of the projection
objective. The other components of the illumination system 610 and the
projection objective 620 (that is to say in particular therefore the EUV
light source, mirror, camera and so forth) in contrast do not have to be
moved for that purpose. The planes of movement in which the two blades
650, 660 are moved extend parallel to and in coplanar relationship with
the mask plane respectively (without the disclosure being restricted
thereto).

[0058] As shown in FIGS. 4 and 6 the blade 650 has openings 651 through
654 corresponding to the desired illumination setting (in the illustrated
example a quadrupole illumination setting) and is moved in a plane of
movement in accordance with the desired angle which is to be set, that
plane of movement in the illustrated example being parallel to the plane
of the mask. Reference "410" denotes a region which is used in the course
of the entire movement of the blade 650 and which as shown in FIG. 4 can
be of a reniform geometry. In that respect for example the illustrated
blade 650 can be suitable for a numerical aperture of NA=0.5 whereas the
further inwardly disposed or smaller reniform regions 411, 412 which are
also shown in FIG. 4 correspond to systems with a smaller pupil or
numerical aperture (for example NA=0.32 for the region 411 and NA=0.25
for the region 412). The reniform region 410 (or 411 or 412 respectively)
thus also represents the region which prior to use of the blade 650 must
be filled with light if or before the blade 650 comes into operation.
That signifies that the optical system is generally designed for a larger
angular region (that is to say a larger numerical aperture).

[0059] FIG. 5 shows the similar situation for the blade 660 which as shown
in FIG. 6 is used in the projection optical system of the mask inspection
apparatus but which has a circular geometry and which in turn is
similarly moved in the pupil plane of the projection optical system of
the mask inspection apparatus (projection objective 620).

[0060] Thus in accordance with the disclosure the structures which are to
be imaged on the mask and which are written in such a way that they
"function" well precisely under the conditions of the projection exposure
apparatus are observed under conditions which as far as possible are the
same in the mask inspection apparatus, wherein account is taken of the
fact that now only a very small field is available and the mask is moved
relative to the projection objective not only in the scanner direction
but also transversely relative thereto (that is to say from left to
right).

[0061] In addition the foregoing situation is described as a somewhat
different view with reference to FIGS. 7a through 7b.

[0062] In that respect in FIG. 7a the optical axis of the projection
optical system of the scanner, which extends perpendicularly to the plane
of the paper, is identified by OAp. The Figure also shows the
scanner slot or the object field 631 which is in the shape of a segment
of a ring and which extends concentrically around the optical axis
OAp (so that the optical axis OAp is at the center point of
curvature of the object field 631 in the shape of a segment of a ring).

[0063] FIG. 7a also indicates both for the left-hand and the right-hand
edge and also for the center of the object field in the shape of a
segment of a ring, the respective configuration of the plane of incidence
`A`, `B` and `C` respectively which each also extend perpendicularly to
the plane of the paper and intersect the optical axis OAp. The
blades 650 and 660 which are disposed in the pupil plane of the
illumination system and the projection objective respectively (that is to
say the projection optical system of the mask inspection apparatus) and
implement the respective configuration of the plane of incidence `A`,
`B`, and `C` according to the disclosure are indicated here as a plan
view.

[0064] FIG. 7b shows a corresponding side view in which OAM denotes
the optical axis of the mask inspection apparatus, wherein the object
field 631 in the shape of a segment of a ring (to be seen in
cross-section here) is disposed at a spacing R from the optical axis of
the projection exposure apparatus and is of the width b. The blades in
the illumination system or projection objective respectively (that is to
say the projection optical system of the mask inspection apparatus) are
here denoted by ASill and ASob respectively.

[0065] The disclosure is thus based on the concept, during the movement of
the mask 630 in the course of mask inspection, to simulate the effective
direction of incidence of the light, which in the embodiment described
here is effected by the blades 650, 660 arranged in the illumination
system 610 and the projection objective 620 (projection optical system)
of the mask inspection apparatus respectively also being moved. That
means, as shown in FIG. 7, that the bundle of rays serving to illuminate
the object field 631 in the shape of a segment of a ring on the mask 630
must "stop" in the left-hand portion of the region 410 at the moment at
which the observed location on the mask 630 is in the left-hand region of
the mask 630 or the object field 631, whereas in corresponding fashion
the bundle of rays passing into the optical imaging system (projection
optical system of the mask inspection apparatus) must "stop" in the
right-hand portion of the region 510.

[0066] While now in the course of mask inspection the respective location
being inspected moves for example from left to right over the mask 630 a
movement of the blades 650 and 660 is effected synchronously and in
mutually opposite directions in such a way that gradually the situation
shown at the left in FIG. 7a changes over to a mirror-image situation as
shown at the right in FIG. 7a, in which the bundle of rays serving to
illuminate the mask 630 stops in the right-hand portion of the region 410
and in a corresponding fashion the optical imaging system stops in the
left-hand portion of the region 510.

[0067] In other words therefore during the scanning procedure the blades
650, 660 are moved over the mask in such a way that depending on the
respective mask location which is currently being inspected, different
directions of the centroid ray are set in relation to the optical axis.
Therefore the blade movement achieves precisely the desired variation in
location of the centroid ray, wherein the illumination setting is
respectively maintained as a consequence of using the same blade.

[0068] It is to be noted in that respect that, with that variation in
location of the centroid ray from one location to another on the mask, no
variation in magnitude of the angle between the incident centroid ray and
the surface normal of the mask is intended, but rather there is a
variation in the direction of that angle or centroid ray (in which
respect it is to be borne in mind that specifying an angle in terms of
magnitude of for example 6° relative to the surface normal of the
mask defines a "cone of directions of incidence", and the variation in
the direction of the centroid ray takes place along that cone).
Accordingly with the variation according to the disclosure preferably the
magnitude of the angle between the surface normal on the mask and the
centroid ray is retained.

[0069] Although hereinbefore reference is made in each case to an
arrangement of the blades in the respective pupil plane of the
illumination system or the projection objective respectively (that is to
say the projection optical system of the mask inspection apparatus) the
disclosure is not restricted to an exact arrangement in the pupil plane.
Rather, having regard to the very small size of the object field in the
microscope, to a good degree of approximation each plane remote from the
mask or the object plane can be viewed as the pupil plane. For ease of
implementation (and without the disclosure being restricted thereto) it
is therefore appropriate for the blades 650, 660 each to be arranged at a
relatively large spacing relative to the mask 630 and as diagrammatically
shown in FIG. 6 to be disposed outside the remainder of the optical
arrangement of the illumination system 610 and the projection objective
620 respectively (that is to say the projection optical system of the
mask inspection apparatus), in which respect in particular there does not
have to be any additional imaging optical means between the blade 650 and
the blade 660 respectively and the illumination system and the projection
objective respectively (projection optical system of the mask inspection
apparatus). The disclosure however is not restricted thereto so that in
other embodiments a further (imaging) optical system can also be provided
between blade and illumination system or projection optical system of the
mask inspection apparatus.

[0070] As regards the respective planes in which the blades 650 and 660
are moved, this can involve the same plane or also mutually coplanar
planes. In addition those planes can be coplanar in relation to the
object plane and the mask plane respectively. The disclosure however is
also not restricted thereto, that is to say under some circumstances it
may also be appropriate for those planes to be tilted in comparison with
the reticle and/or in relation to each other.

[0071] A mathematical description of the variation in the centroid ray in
dependence on the location on the mask can be as follows: if x and y are
used to denote the co-ordinates of the mask plane and z is used to denote
the co-ordinate perpendicular to the mask plane, and if in that respect x
is used to denote the co-ordinate along which scanning is effected, the
active surface of the mask extends in the y-direction from -b/2 through
+b/2, wherein b denotes the scanner slot width (which can be of a value
by way of example of 104 mm).

[0072] The direction of the centroid ray of the illumination bundle of
rays can be defined by way of the vector (cx, cy, cz),
wherein cx2+cy2+cz2=1 and cx denotes
the projection on to the x-axis, and so forth (=so-called "directional
cosine representation" with standardized directional vector c).

[0073] With a ring field radius R (defined as the spacing of the center of
the ring field from the optical axis of the projection objective) the
resulting formulae are as follows:

wherein cz corresponds to the cosine of the angle of incidence which
can involve typical values of at the present time between 6° and
about 8°-9° or thereabove (for higher numerical apertures).

[0074] An example of calculation for the edge of the mask can be as
follows: maximum value for y: yMax=52 mm, R (=ring field radius)=100
mm and cx=0.089, cy=0.054 and cz=cos(6°)=0.995.

[0075] The angle α which the plane of incidence (formed by the ray
which is incident on the mask from the illumination system and which is
emergent, that is to say entering in the projection objective) includes
with the y=0 plane is given by:

tan ( α ) = c y c x ( 3 ) ##EQU00003##

or also by

sin ( α ) = y R ( 4 ) ##EQU00004##

wherein in the foregoing example α=31.2 degrees.

[0076] In accordance with a further aspect of the disclosure account is
also taken of situations in which the illumination pupil in its
brightness distribution has substructures deviating from an idealized
form. Such a substructure can be governed in particular by using in the
illumination arrangement a honeycomb condenser producing a multiplicity
of light passages which are superposed in the plane of the mask. Those
light passages now do not completely fill the pupil so that it is
possible to see individual spots or "illumination peaks".

[0077] FIG. 8 shows in that respect an example of a typical EUV pupil at
two different field points. The numbers respectively shown in the white
circles represent the different levels of brightness of the pupil parcels
and their field dependency.

[0078] To take account of the above-described situation it is also
possible in accordance with the disclosure to produce not only idealized
illumination distributions (for example with homogeneous dipole or
quadrupole settings) in the aperture plane of the mask inspection
apparatus, but also to preserve the complete "substructured" illumination
distribution within the aperture plane.

[0079] Reference will now be made hereinafter to FIGS. 9 through 11 to
describe embodiments which serve to reproduce the above-described
parcellings of the pupil illumination including the fluctuations in
intensity in mask inspection.

[0080] For that purpose in some embodiments, instead of a conventional
one-part blade, a blade arrangement comprising at least two blades (in
further embodiments therefore also three, four or more blades) is used,
wherein those blades are so-to-speak "connected in series" and are of
such a design configuration that relative displacement and/or rotation of
the blades makes it possible to adjust the effective opening of the pupil
parcels and thus the integral brightness or light efficiency thereof.

[0081] Referring to FIGS. 9a and 9b, the blade arrangement is made up of
two blades 910, 920 which are arranged in succession in the light
propagation direction and which each have three holes 911-913 and 921-923
respectively (for example a respective hole for each pupil parcel),
wherein however both blades 910, 920 have holes of differing shape. Thus
the first blade 110 which is shown only diagrammatically and by way of
example has three circular holes 911-913, whereas the second blade 920
has a circular hole 922 between two elongate holes 921 and 923. The
individual blades of the blade arrangement do not necessarily have to be
arranged in directly successive relationship.

[0082] As shown in FIGS. 9c, 9d and 9e a variation in the effective hole
size for each pupil parcel and thus adjustment of the integral brightness
or light efficiency can be achieved by relative displacement of the
blades 910, 920 in a horizontal direction. If for example the size of the
holes 911-913 in the first blade 910 is quantified with a (notional)
value 3, then for example the hole size can be viewed as a function of
the horizontal displacement as specified in Table 1:

TABLE-US-00001
TABLE 1
Displacement Displacement
towards the left No displacement towards the right
Top left 3 3 2
Top center 2 3 2
Top right 2 3 3

[0083] Thus in accordance with the disclosure a substructuring as
described hereinbefore of the pupil illumination in the mask inspection
apparatus can be afforded.

[0084] In a further embodiment diagrammatically illustrated in FIG. 10 it
is also possible to provide a variable hole size and thus a variation in
the effective opening of the pupil parcels and thus the integral
brightness or light efficiency, by at least one blade of at least two
blades 950, 960 having at least one apodising arm (here the second blade
960 shown in FIG. 10b) which partially covers the hole opening and thus
leads to an effective reduction in the size of the hole. Thus the
effective hole size is altered passage-wise by relative displacement of
the second blade 960 relative to the first blade 950 in FIG. 10a, by the
hole size in the first blade 950 being reduced by the area of the arm
covering it.

[0085] FIG. 11 is a merely diagrammatic view showing further possible
configurations of an arm on a blade 970.

[0086] In a further embodiment a blade arrangement for variable adjustment
of the intensity in the above-described subpupils or substructures can
also be implemented in a (single-stage) blade having a multiplicity of
variably adjustable iris blades which are adjustable individually or in
their entirety by suitable actuators.

[0087] It will be appreciated that the above-described concept of
providing the complete "substructured" illumination distribution within
the mask inspection apparatus can also be implemented independently of
the concept according to the disclosure of varying the direction of the
centroid ray in dependence on the location on the mask. In accordance
with a further aspect therefore the disclosure also concerns a mask
inspection apparatus in which a substructure of the illumination
distribution is afforded, preferably by using a suitable blade
arrangement, without at the same time the variation in the direction of
the centroid ray in dependence on the location on the mask also being
implemented.

[0088] Even if the invention has been described by means of specific
embodiments numerous variations and alternative embodiments will be
apparent to the man skilled in the art, for example by the combination
and/or exchange of features of individual embodiments. Accordingly the
man skilled in the art will appreciate that such variations and
alternative embodiments are also embraced by the present invention and
the scope of the invention is limited only in the sense of the
accompanying claims and equivalents thereof.